The present invention relates generally to integrated circuit design and fabrication systems. More specifically, the invention relates to mechanisms for generating and inspecting reticles.
Generation of reticles and subsequent optical inspection of such reticles have become standard steps in the production of semiconductors. Initially, circuit designers provide circuit pattern data, which describes a particular integrated circuit (IC) design, to a reticle production system, or reticle writer. The circuit pattern data is typically in the form of a representational layout of the physical layers of the fabricated IC device. The representational layout typically includes a representational layer for each physical layer of the IC device (e.g., gate oxide, polysilicon, metallization, etc.), wherein each representational layer is composed of a plurality of polygons that define a layer""s patterning of the particular IC device.
The reticle writer uses the circuit pattern data to write (e.g., typically, an electron beam writer or laser scanner is used to expose a reticle pattern) a plurality of reticles that will later be used to fabricate the particular IC design. A reticle inspection system may then inspect the reticle for defects that may have occurred during the production of the reticles.
A reticle or photomask is an optical element containing transparent and opaque, semi-transparent, and phase shifting regions which together define the pattern of coplanar features in an electronic device such as an integrated circuit. Reticles are used during photolithography to define specified regions of a semiconductor wafer for etching, ion implantation, or other fabrication process. For many modern integrated circuit designs, an optical reticle""s features are between about 1 and about 5 times larger than the corresponding features on the wafer. For other exposure systems (e.g., x-ray, e-beam, and extreme ultraviolet) a similar range of reduction ratios also apply.
Optical reticles are typically made from a transparent medium such as a borosilicate glass or quartz plate on which is deposited on an opaque and/or semi-opaque layer of chromium or other suitable material. However, other mask technologies are employed for direct e-beam exposure (e.g., stencil masks), x-ray exposure (e.g., absorber masks), etc. The reticle pattern may be created by a laser or an e-beam direct write technique, for example, both of which are widely used in the art.
After fabrication of each reticle or group of reticles, each reticle is typically inspected by illuminating it with light emanating from a controlled illuminator. An optical image of the reticle is constructed based on the portion of the light reflected, transmitted, or otherwise directed to a light sensor. Such inspection techniques and apparatus are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif.
During a conventional inspection process, the optical image of the reticle is typically compared to a baseline image. The baseline image is either generated from the circuit pattern data or from an adjacent die on the reticle itself. Either way, the optical image features are analyzed and compared with corresponding features of the baseline image. Each feature difference is then compared against a single threshold value. If the optical image feature varies from the baseline feature by more than the predetermined threshold, a defect is defined.
Although conventional reticle inspections provide adequate levels of detection accuracy for some applications, other applications require a higher sensitivity or lower threshold value (for identifying defects) while other applications require less stringent, higher threshold levels. Since conventional inspections analyze all features of a given type of reticle with the same threshold and analysis algorithm, some features are inspected too stringently while other are not inspected stringently enough.
For example, critical features of an integrated circuit typically include gate widths of the semiconductor transistor devices. That is, a gate width on the reticle needs to produce a corresponding gate width on the circuit pattern within a relatively small margin of error in order for the fabricated IC device to function properly. If the threshold is set too high, these critical gate areas are not checked adequately enough. Conversely, other features, such as the widths of the interconnections between gate areas, do not affect the function of the integrated circuit as much as the gate area width and, thus, do not need to be inspected as stringently as other features, such as gate width. If the threshold is set too low, too many of these noncritical features may be defined as defects such that the inspection results are difficult to interpret and/or computational resources are overloaded.
In sum, conventional inspection systems waste valuable resources by inspecting regions of the reticle too stringently, and not reliably inspecting other regions stringently enough. In other words, the above described inspection system fails to reliably detect defects within critical areas and inefficiently inspects noncritical regions where somewhat larger defects will not present a problem. Conventional inspection systems and techniques are unable to distinguish between critical and noncritical areas of the reticle. Put in another way, conventional design documentation (e.g., electronic reticle or integrated circuit information) fails to adequately transmit the IC designer""s intent regarding the circuit tolerance and resulting IC device dimensions to reticle writer systems, reticle inspection systems, and ultimately wafer inspection systems.
What is needed is improved IC documentation and apparatus for efficiently and reliably writing and inspecting reticles and wafers for determining whether a reticle has defects in critical areas, as well as noncritical areas.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for transmitting the designer""s intent to the pattern generator, the reticle inspection system and ultimately to the wafer inspection system and for efficiently and reliably inspecting reticles. The present invention provides mechanisms for flagging critical or noncritical regions of an IC circuit pattern data base. Other design flow procedures, such as reticle production and inspection and IC device fabrication, may then be based on the flagged critical or noncritical areas of the IC circuit pattern database.
In one embodiment, a circuit design for use with electronic design automation (EDA) tools in designing integrated circuits is disclosed. The circuit design is stored on a computer readable medium and contains an electronic representation of a layout pattern for at least one layer of the circuit design on an integrated circuit. The layout pattern includes a flagged critical region which corresponds to a critical region on a reticle or integrated circuit that is susceptible to a special inspection or fabrication procedure. The flagged critical region contains a flag that is readable by an inspection or fabrication system. In a preferred embodiment, the circuit design is reusable.
In one aspect of the circuit design, the special analysis is performed during a technique selected from the group consisting of reticle inspection, reticle production, integrated circuit fabrication, and fabricated integrated circuit inspection. In another aspect of the invention, the circuit design includes (i) a base representation containing the entire layout pattern without denoting the flagged critical region and (ii) a shadow representation that flags the critical region without denoting the entire layout pattern. In one embodiment, both the base and shadow representations are configured to together provide instructions for generating or inspecting a single reticle.
In another aspect of the invention, a method of producing a reticle for an integrated circuit device is disclosed. An electronic representation is provided to a reticle producing system. The electronic representation has a flagged critical region that indicates to the reticle producing system that an associated critical region of the reticle requires a special production technique. A reticle based on the electronic representation is produced. The critical region of the reticle associated with the flagged critical region of the electronic representation is produced via the special production technique and other regions of the reticle are produced via a normal production technique. A computer readable medium for storing computer readable code that implements the above reticle production method is also described.
In another method aspect of the invention, a method of inspecting a reticle for defining a circuit layer pattern is provided. The reticle has a special analysis region associated with a critical region and a normal analysis region associated with a normal region. An electronic representation of the circuit layer pattern is provided. The representation has a normal region of the pattern and a flagged critical region of the pattern. A test reticle image of the reticle is provided. A baseline representation containing an expected pattern of the test reticle image is also provided. The test reticle image is compared to the baseline representation such that (i) regions of the test reticle image and the baseline representation corresponding to the normal analysis region of the reticle are compared via a normal analysis and (ii) regions of the test reticle image and the baseline representation corresponding to the special analysis region of the reticle are compared via a special analysis.
In a preferred embodiment, the comparison further includes determining whether a special parameter of the special analysis region is within a first threshold of an associated parameter of the baseline special analysis region. The comparison also may include determining whether a normal parameter of the special analysis region is within a second threshold of an associated parameter of the baseline normal analysis region. A computer readable medium for storing computer readable code that implements the above reticle inspection method is also described.
In another apparatus aspect, a circuit design for use with electronic design automation (EDA) tools in designing integrated circuits is disclosed. The circuit design is stored on a computer readable medium and contains an electronic representation of a layout pattern for at least one layer of the circuit design on an integrated circuit. The layout pattern includes a flagged noncritical region which corresponds to a noncritical region on a reticle or integrated circuit that is susceptible to special inspection or fabrication procedure. The flagged noncritical region contains a flag that is readable by an inspection or fabrication system.
In an alternative embodiment, the special inspection procedure includes using a low stringency threshold to compare the noncritical region of the reticle or integrated circuit to the flagged noncritical region of the layout pattern and using a normal stringency threshold to compare a normal region of the reticle or integrated circuit that is outside of the flagged noncritical region to a normal region of the layout pattern that is outside the flagged noncritical region of the layout pattern.
The present invention has several advantages. For example, the present invention allows more than one type of inspection, an enhanced or special inspection and a normal inspection. This feature results in significant improvements to the inspection process by providing flexible inspection techniques for various applications. Additionally, by facilitating an enhanced inspection for certain critical areas of the reticle and/or wafer, the present invention may contribute to significant increases in device yield. That is, as IC device speed increases, structures must meet tighter tolerance requirements, and the present invention provides mechanisms for meeting these tighter tolerance requirements economically, that is, without requiring all features to be patterned and inspected to the tighter tolerance requirements.